Something we cover on this blog and in our help over and over again is the importance of having explicit permission before you email someone. However, just having that explicit permission is not enough. Recently the FTC Spam Summit 2007 was held in Washington, subtitled "The Next Generation of Threats and Solutions".
According to Jordan Cohen of Epsilon , some of the top ISPs and email providers are moving to a looser definition of spam than is currently held. According to Jordan, a Yahoo representative said:
Operationally, we define spam as whatever consumers don't want in their inbox.
Other influential people were heading in the same direction - it's not enough that someone originally asked you for email, they have to actually keep wanting to receive it. Getting someone onto your list is not the end of your job. You have to work at keeping them happy too.
So what does that mean for you as a designer? One key point is to recognise that some percentage of people on your list probably don't want to be on it. You should make it much easier for them to unsubscribe than to mark it as spam. So don't hide your unsubscribe link in tiny font or bury it in a paragraph.
Every campaign needs to be absolutely relevant for your subscribers. Secondly, keep encouraging your clients to concentrate on list quality instead of list size. Work on making the content more valuable to your subscribers so they want to keep getting it. That's your best ongoing protection against spam complaints.